A generic anti-rotation device, in this case, for a coupling piston of a switchable roller tappet, is disclosed in FIG. 6 of EP 1 149 989 B1 (which is not reproduced here). This document discloses an open circular wire which is seated on the outer peripheral wall of the inner element and extends under the flattened portion of the respective coupling piston. In addition, FIG. 4 of this document also shows an anti-rotation device for the coupling piston, said anti-rotation device being constituted by a pin seated in longitudinal grooves.
A drawback of the first-mentioned circular wire variant is its inadequate rigidity for enabling a displacement limitation and an anti-rotation feature. Thus, there is a danger of an excessive twisting of the coupling piston, so that one of the piston edges comes to be situated under the coupling surface, which leads to wear or makes coupling impossible. Moreover, the circular wire can expand to an undesired extent. Furthermore, assembly has to be done in the right direction.
The aforesaid second, pin variant requires a great deal of assembly work and leads to high fabrication costs.